1. Field of the Invention
The invention relates to techniques of forming an image on a recording medium (e.g., a recording paper) with its lightness be adjustable, and more particularly to techniques of converting image data (e.g., data indicative of an image scanned or captured by a color scanner) of additive primary colors (e.g., three primary colors) into recording data of subtractive primary colors (e.g., a combination of three primary colors and a black color), and forming an image on the recording medium using the recording data.
2. Description of the Related Art
Color copiers, and facsimile machines and digital multi-function apparatuses each having a copying function are known. Typically, such a type of an apparatus is constructed to include: a color scanner (an image reader or an image capture device); an image processor; and a color printer (an imager or an image recorder).
The color scanner is adapted to capture the image of an original document, by separating the image into color components of R (red), G (green), and B (blue), which are additive three primary colors, and by producing image data indicative of the color components. The image data will be also referred to as “captured-image data.”
The image processor is adapted to produce image data indicative of color components of C (cyan), M (magenta), and Y (yellow), which are subtractive three primary colors, and K (black), from the image data (captured-image data) produced by the color scanner. The image data produced by the image processor will be also referred to as “recording data.”
The color printer is adapted to form an image on the recording medium, using four different colored colorants of C, M, Y, and K colors, based on the recording data of C, M, Y, and K color components produced by the image processor.
Such a type of an image forming apparatus including the above-described color scanner, image processor, and color printer is operated such that an original document is captured by the color scanner to produce captured-image data (image data indicative of R, G, and B color components), the captured-image data is converted into the recording data indicative of C, M, Y, and K color components, and then, the color printer outputs a copy image of the original document, using the recording data of these four color components.
Color copiers and multi-function apparatuses incorporating a copying function are typically provided with as one of standard keys a lightness-adjustment key for allowing the user to request or instruct the lightness of an image formed on a recording paper (hereinafter, referred to as “copy image output”) to be adjusted.
The lightness-adjustment key is constructed as a member manipulated by the user for changing a set value of the lightness value of a copy image output, relative to a standard lightness value (e.g., a default value for the lightness), at given intervals, in multiple steps, in a selected one of a plus direction to brighten the copy image output, and a minus direction to darken the copy image output. The user is allowed to adjust in lightness a copy image output as a whole, to achieve a desired lightness, either for brightening or for darkening, depending on the user's manipulation via the lightness-adjustment key.
More specifically, the aforementioned image processor is constructed to include, as illustrated in FIG. 10, a first gamma corrector 101; a color converter 102, a UCR (Under Color Removal) processor 103; and a second gamma corrector 104.
The first gamma corrector 101 is for use in gamma-correcting the image data of R, G, and B color components delivered from the color scanner. The color converter 102 is for use in producing from the image data of R, G, and B color components gamma-corrected by the first gamma corrector 101, image data of C, M, and Y color components, through color conversion. The UCR processor 103 is for use in producing from the produced image data of C, M, and Y color components, image data of a K color component. The second gamma corrector 104 is for use in gamma-correcting the image data of C, M, Y, and K color components delivered from the UCR processor 103.
In the thus-constructed image processor, a gamma characteristic used in the first gamma corrector 101 is made variable depending on the set value of lightness established to the user's operation via the lightness-adjustment key, as illustrated in graph in FIG. 11, and the captured-image data is gamma-corrected according to the gamma characteristic conforming with the set value of lightness established to the user's operation via the lightness-adjustment key, resulting in adjustment in lightness of a copy image output.
More specifically, as illustrated in FIG. 11, in the above image processor, there are stored data of a gamma characteristic curve for achieving the standard lightness value, which forms the basis of the following curves; data of a gamma characteristic curve for achieving the lightness adjustment in the minus (darkening) direction; and data of a gamma characteristic curve for achieving the lightness adjustment in the plus (brightening) direction.
In the image processor, an available gamma characteristic curve is sequentially modified by selecting these curves to the user's selective action of pressing the lightness-adjustment key, resulting in adjustment in lightness of a copy image output.
For darkening a copy image output, there is established through the above selective operation a gamma characteristic curve, i.e., a relationship in lightness between an input value and an output value defined such that increments of the output value with respect to the input value (i.e., the slope of the gamma curve) are lower in the region with the input value being lower, while increments of the output value with respect to the input value are higher in the region with the input value being higher.
On the other hand, for brightening a copy image output, there is established through the above selective operation a gamma characteristic curve defined such that increments of the output value with respect to the input value are higher in the region with the input value being lower, while increments of the output value with respect to the input value are lower in the region with the input value being higher.
The above technique, since is originated for adjusting lightness or brightness, is considered to be a technique of adjusting the lightness “L” when viewed in an L*a*b* color space. With this in mind, conventionally, the adjustment is performed at only the first gamma corrector 101 on a reading side of the instant apparatus, independent of the second gamma corrector 104 on a recording side of the instant apparatus.
On the other hand, as disclosed in Japanese Patent Publications No. HEI 10-79888 and No. 2003-46779, for example, there is known in the field of an image capture device such as a digital camera and a video camera, a technique of modifying a gamma characteristic for use in a gamma correcting processing for image data of a captured image, thereby varying the density of an output image.